Caller ID has been a popular service provided by public wired and mobile phone networks. Wired phones or mobile phones with caller ID feature are also popularly available at stores almost everywhere. The caller ID feature not only allows users to screen incoming calls, but also grants users the advantage of direct callback capability with the caller ID stored. This saves time and effort for phone number look up and phone number key in.
In spite of the recent out booming of network phone development, there is one general limitation, however, to the use of a current network phone. A PSTN user cannot take the advantage of the caller ID feature to call back a user from a network phone. Unless with the direct inward dialing (DID) service provided by an intelligent phone network, a user within a phone network cannot be directly reachable by one-stage-dialing. Such limitation also exists in the phone communication between users in a conventional private phone system and the users from a PSTN.
Currently, private telephone networks have been widely deployed in most companies, enterprise phone networks, and internet phone networks. A typical private enterprise phone network uses a private branch exchange (PBX) or a key system to access to each employee's extension phone through internal switching circuitry. A PBX usually connects to the PSTN with a few external leased lines, by which each extension phone within the PTN can access to the users in the PSTN. FIG. 1a illustrates an environment for such an enterprise PTN.
Referring to FIG. 1a, as an example, a PTN user A at extension 123 attempts to call a PSTN user B at 2705-8167. The general procedure is as follows. User A picks up the phone set and dials 0 or 9 to inform the PBX of an outgoing call; the PBX dynamically chooses an unused external leased line, (for example the line at number 2793-7887), and connects to the internal line at extension 123 for user A; user A hears the dial tune from the PSTN; user A then dials user B's external number 2705-8167. When user B's phone rings, the caller ID that the user B sees on the phone display is the external line number, 2793-7887.
Under such circumstances, regardless if the call from user A is picked up by user B or not, the caller ID 2793-7887 is logged in user B's phone. Sometime later, when user B tries to call back user A with this caller ID, user B cannot directly reach user A's extension 123. User B's call is first picked up by the PBX, then user B gives the PBX further instruction with the internal extension 123 or asks the main dispatcher to redirect the call to extension 123. This procedure is referred to as two-stage dialing.
The two-stage dialing limitation for a PTN system is either due to the private number plan used in a PTN other than the public numbering used in the PSTN, or due to the number of external leased lines which connect a PTN to a PSTN is less than the number of users in the PTN for economic reasons.
To simplify the calling procedure to reach a PTN user to one-stage dialing, the conventional PSTN and PBX manufactures use the suffix three or four digits of a phone number as the internal extension identifier to provide the direct access to a PTN number. When a PSTN user dials a number, such as 2793-0123, with the prefix digits, 27930, the PSTN recognizes it as the main line of an enterprise PTN and connects the call to the PBX of the enterprise PTN and provides the suffix 3 or 4 digits, such as 123, to the PBX. The PBX can direct the incoming call to the designated extension 123 without the need to pick up the call and thus accomplishes a one-stage dialing.
Even with such one-stage direct dialing capability, when user A makes a call to a PSTN user B at 2705-8167 through the internal extension phone at 123, user A still goes through the same fore mentioned procedure, in which the PBX dynamically selects an unused external line to connect to the PSTN. Eventually the caller ID displayed on user B's phone is still the leased line number, 2793-7887, instead of user A's direct dialing number, 2793-0123. When user B uses this caller ID to call back user A, the process still needs two-stage dialing to complete the call.
With the prevailing trend of network phones, enterprise PTNs start using network phones and local area network to replace the conventional telephony-based analog phones and telephone wiring. FIG. 1b illustrates a PTN environment with network phone architecture, wherein the conventional private branch exchange (PBX) is replaced by either an internet protocol private branch exchange (IP PBX), or a media gateway. An IP PBX or a media gateway still connects to the PSTN with a few external leased lines.
When a PTN user A (with a unique identifier name), via a network phone (with a unique network address), makes a call to a PSTN user B at 2705-8167, the IP PBX or the media gateway recognizes the destination number as an PSTN number and dynamically chooses an unused leased line, (for example the line at number 2793-7887), to setup the call. Eventually the caller ID displayed at the PSTN user B's phone is still the number for the external leased line, 2793-7887. When user B uses this caller ID to call back PTN user A, the frustration of two-stage dialing still exists. The PTN user identifier name cannot be passed to the PSTN user when a PTN user (user A) calls a PSTN user (user B).